Cadmium is a very interesting element and the properties we use from it are very telling as to why we loved it so much. Did you know, cadmium is not outlawed, even though it is a known carcinogen, and is on the ROHS list? It has not been totally forbidden, the reason being that the properties used have not been replaced. Currently cadmium is seeing a resurgence, mainly because we have started to understand why things worked with cadmium and its properties. In the past cadmium specifications came in three main plating specifications;
A. Cadmium electroplating 8µms with a yellow chromate or ptfe. (normal fastener spec)
B. Cadmium electroplating 8µms with a zinc electroplating and a ptfe. (Added shelf life spec)
C. Nickel plating with a cadmium electroplated top, that was then diffused at more than 400 C, to form an amorphous Ni-Cd plating that was protective against stress cracking. (This is the spec that is still used today on airplane landing gear and is replated with cadmium as a touch up service.)
Properties of Cadmium
1. Open Circuit Potential of cadmium is very important and much of its properties are telling from this one difference. OCP = -.850mV in a 3% Sodium Chloride.
Chromate has been the silent partner of coating and platings for decades. Whether it was Hexavalent Chromate, Trivalent Chromate, or Bi-Chromate. There are many other variants and mixed versions. The one thing they have in common , is they all have a level of the good stuff in them (Hex). This is the main reason for outlawing all chromates rather than just the hexavalent by itself. This is also the reason why some trivalent work better than others. Trivalent, works very well on zinc applications. I have transformed hexavalent chrome to trivalent chrome at 650 F (PMT). I have even written instructions to test for hexavalent or trivalent chromate.
Are we making a big mistake getting rid of chromate in general? (Yes.) Even though I am looking to the future and have substitutes for the chromate top coat, chromate was very good at protecting from corrosion. It worked in tandem with many platings and was part of the dip spin mixture in coatings.
A. Cadmium electroplating 8µms with a yellow chromate or ptfe. (normal fastener spec)
B. Cadmium electroplating 8µms with a zinc electroplating and a ptfe. (Added shelf life spec)
C. Nickel plating with a cadmium electroplated top, that was then diffused at more than 400 C, to form an amorphous Ni-Cd plating that was protective against stress cracking. (This is the spec that is still used today on airplane landing gear and is replated with cadmium as a touch up service.)
Properties of Cadmium
1. Open Circuit Potential of cadmium is very important and much of its properties are telling from this one difference. OCP = -.850mV in a 3% Sodium Chloride.
- this means that it is less than 300mV difference from low alloy bolting (-.675mV) . Zinc plating is at -1.100mV. The further away from one another you get the greater the galvanic dissimilarity becomes. 300mV is the said limit not to cross by the Navy. NASA due to criticality uses 150mV limits.
- resistivity of cadmium plating is greater than that of say, zinc or zinc-nickel plating. Meaning it acts more as a barrier to current. Example of this is the "impressed current" used in sub sea cathodic protection which uses -900mV, -1.100mV and even -1.200mV.....
- Cadmium oxide, Aluminum oxide, Chromium oxide all react super fast when a scratch occurs in the thin film. Re-oxidizing the surface where the scratch was made with oxide crystalline structure. Zinc oxides form quickly, but also form zn-fe oxides as well allowing the iron oxide to be brought to the surface.
Chromate has been the silent partner of coating and platings for decades. Whether it was Hexavalent Chromate, Trivalent Chromate, or Bi-Chromate. There are many other variants and mixed versions. The one thing they have in common , is they all have a level of the good stuff in them (Hex). This is the main reason for outlawing all chromates rather than just the hexavalent by itself. This is also the reason why some trivalent work better than others. Trivalent, works very well on zinc applications. I have transformed hexavalent chrome to trivalent chrome at 650 F (PMT). I have even written instructions to test for hexavalent or trivalent chromate.
Are we making a big mistake getting rid of chromate in general? (Yes.) Even though I am looking to the future and have substitutes for the chromate top coat, chromate was very good at protecting from corrosion. It worked in tandem with many platings and was part of the dip spin mixture in coatings.